The present invention is an improvement to the method of detecting creatinine in samples of body fluid. There is disclosed in U.S. Pat. No. 5,374,561 a method for the detection of creatinine in urine which involves contacting a urine sample suspected of containing creatinine with cupric ions, a hydroperoxide and an oxidizable dye which provides a colored response in the presence of oxygen free radicals and a hydroperoxide. In this assay, the first step involves the formation of a Cu.sup.++.creatinine chelated complex. The oxidizable dye is oxidized by the transfer of an electron therefrom to the Cu.sup.++.creatinine complex to provide the non-reactive Cu.sup.+.creatinine form which is regenerated to Cu.sup.++.creatinine by the loss of an electron to the hydroperoxide. This assay works well in the absence of hemoglobin and ascorbate, but in the presence of hemoglobin and/or ascorbate, which are normally present in clinical urine samples, the precision of the assay is affected due to the tendency of hemoglobin and/or ascorbate to oxidize the dye which results in false positive results. In addition, the redox dye can auto-oxidize over time in the presence of copper ion thereby reducing the shelf life of this type of assay. Accordingly, it would be desirable and it is an object of the present invention to provide an improved reagent system for the detection of creatinine in a fluid test sample in which the tendency of hemoglobin and/or ascorbate in the test sample to cause falsely positive results and the auto-oxidation of the redox dye is reduced.